1. Field of the Invention
The present invention relates in general to geolocation technology using radio waves, and, more particularly, to a system and method of precise geolocation of moving or fixed transmitters using multiple observers.
2. Description of the Prior Art
A technology known as Kinematic Ambiguity Resolution (KAR) has been developed for precision tracking using global positioning system (GPS) receivers that receive signals from many GPS satellites. The fundamental approach is to use carrier phase observations of many GPS satellites to determine the location of a static or moving GPS receiver. Under favorable circumstances, the location of a moving GPS antenna can be fixed to centimeter accuracy within seconds.
At a fundamental level KAR involves measuring range to GPS satellites using carrier phase. Extremely precise measurements can be made on carrier phase, usually better than 1/100 of a wavelength. Unfortunately these phase measurements have an ambiguity as to which cycle one is measuring. For example the GPS C/A code has a wavelength of 19 cm. One might be able to measure the carrier phase and determine the exact location of the wave to 1 mm, but with an ambiguity of 19 cm. If one knew the transmission time of the measured cycle, the location of the transmitter, and all propagation effects then the phase measurement determines the range to the satellite with an ambiguity of 19 cm. So the GPS receiver's antenna phase center is known to be on one of many spherical shells centered on the satellite.
Typically the time at the GPS receiver is not precisely known, so in order to fix a three-dimensional position in space, one must measure the signal from at least four GPS satellites. This allows solution of three position coordinates and time of the GPS receiver. When observing the time of arrival of the PN signal modulated onto the carrier, this is sufficient to fix a unique location. However this location accuracy is limited by the precision with which the modulation time of arrival can be measured. And this is, in turn, limited by the bandwidth of the modulation. GPS signals have modulation bandwidths of 1 and 10 MHz, so one can make relatively accurate measurements of the location based on timing the modulation alone. This allows one to estimate the location of the GPS antenna to the level of meters.
Making measurements on the carrier phase instead of the modulation has the potential of improving the position fix accuracy from meters to millimeters. Unfortunately, because of the ambiguity in carrier phase measurement, there are a multiplicity of locations where the antenna might be. These possible locations form a lattice pattern with spacing typically on the order of a wavelength apart. The details of this lattice vary depending upon the geometry of the GPS satellites. If one can resolve which of the possible locations is the true location, then determining location to millimeter accuracies becomes merely a matter of eliminating all the biases may creep into the entire system. Examples of biases include errors in satellite position, changes in the phase center of the GPS antenna with angle, local multipath reflections, tropospheric propagation effects, and ionospheric propagation effects. We will discuss elimination of biases at a later time. For now we will focus on the difficult problem of resolving the ambiguities.
If the GPS receiver is fixed, a possible way to resolve the ambiguities is to wait while a GPS satellite moves to present a different geometry. One might start with a lattice of possible GPS locations and with successive measurements (with different GPS geometries) compute new lattice. The lattice point corresponding to the true location will remain static while the other points move. With enough satellite geometry change, the true location will become apparent. Another approach is to choose an army of lattice points that best fit the observed data. Then one calculates the residual rms phase error, the rms of (measured phase−calculated phase) summed over all satellites. Eventually the true location's rms phase error will be much better than the rms phase error of the incorrect ambiguities.
Unfortunately GPS satellites move slowly. Fortunately there are a lot of GPS satellites, and each transmits two different wavelengths. Observing with two wavelengths simultaneously allows one to eliminate ambiguities. In the case of GPS, the ambiguities of both carrier wavelengths are spaced at a distance where the shorter wavelength has exactly one more cycle than the longer wavelength. This distance, known as the wide-lane ambiguity, is about 0.9 meter. Observing more than four GPS satellites serves to improve precision. A fifth satellite with a good geometry will add phase measurement data that matches the true location, and not many others. Unfortunately, measurement errors create uncertainty as to whether a particular ambiguity matches the data or not. Usually the procedure is to determine the lattice of possible locations, and calculate the rms phase error associated with each one. If one location produces much better rms errors than all the others (e.g., by a factor of three or more), then this location is deemed to be correct.
In order to successfully perform KAR, one must have a data set with little measurement error and bias. An approach to obtaining this data set has been to place a second GPS receiver near to the location where KAR is being performed. This receiver is used as a Reference station. It observes each of the satellites being collected by the GPS receiver and saves measurements on the code and especially the carrier phase. Even though the two receivers are separated, many of the errors due to satellite position, atmospheric propagation, and ionospheric propagation appear in common at both receivers. So by subtracting the Reference's data from the Rover's data, a new data set with less error is generated. With GPS, KAR works best when the reference receiver is within 10 km of the rover's position. As separation is increased, separate propagation paths through different parts of the atmosphere and ionosphere reduces the amount of error that is correctable.
Geolocating a non-cooperative transmitter observed by multiple collectors differs significantly from the GPS problem previously described. In the first instance, a complication is presented by the fact that transmitter waveforms are commonly not designed for the task at hand. Instead of measuring the phase of transmitter carriers locked to stable Rb or Cs oscillators, one is faced with a signal that may be drifting in frequency and that may not even have a carrier. In addition, the signal may be narrow-band modulated so that instead of Time-Difference-of-Arrival (TDOA) measurements on the signal constraining the search region needing to resolve possible ambiguities to perhaps 10 meters, the search region may be 1 km or more.
The typical approach to geolocating such a signal is to cross-correlate the signal seen at different collectors. The peak of the cross-correlation function in TDOA is used to define hyperboloids on which the transmitter must lie. The rate of change in phase of the cross-correlation function's peak is used to define the Frequency-Difference-of-Arrival (FDOA). FDOAs also define surfaces on which the transmitter must lie if the transmitter isn't moving. However, if the transmitter is moving, then FDOA measures a composite of the transmitter and collector velocity. It is commonly thought that phase vs. time measurements cannot determine a transmitter's location if it is moving.
In light of the foregoing, a need exists for a precise system and method of geolocating a transmitter which is observed by collector devices which are fixed or moving. In addition, a need exists for a geolocation system and method which serves to alleviate the problems of geolocating a non-cooperative transmitter as described.